The present invention relates generally to protective circuits for sheathed electrical resistance units. More particularly, the invention relates to circuits for reliably interrupting a ground fault in the form of an arcing short which may occur between the heating element and the grounded outer sheath of such a heating unit.
Sheathed electrical resistance heating units have been employed for many years in various products including, for example, dishwashers and electric ranges. Such heating units generally comprise a heating element in the form of a spiralled, electric resistance wire encased in an elongated, ceramic-filled, metallic outer sheath which is electrically conductive. The ceramic material transmits heat but, in its normal state, is an electrical insulator. Thus, the outer sheath becomes thermally hot but normally remains electrically insulated from the heating element. A suitable ceramic material is magnesium oxide. Sheathed electrical resistance heating units of this general type are described in U.S. Pat. No. 2,094,480 to Vogel; and U.S. Pat. No. 3,592,771 to Vedder et al.
In the operation of such heating units, particularly in dishwashers, the terminals of the heating element are connected to a power source, for example, a 60 Hz, 120 volt household AC power line. A 120 volt heating unit is normally connected between the hot or power line and the neutral line. Normally, the outer conductive sheath is grounded.
Though such heating units for the most part perform satisfactorily, one failure mode which is possible in such a heating unit is associated with a breakdown in the insulation qualities of the magnesium oxide separating the heating element from the outer sheath, accompanied by the formation of a current path between the element and the outer sheath. For reasons not fully presently understood, occasionally a discontinuity develops in the heating element resulting from a physical separation or break in the element itself. This break in the element creates an arc at the discontinuity between the two ends created by the break. The intense heat from the arc rapidly creates a plasma condition in which some of the magnesium oxide material in the vicinity of the arc is vaporized and some melted. The nature of the magnesium oxide is such that in its normal solid state it provides a very high resistance; however, in its molten liquid and gaseous plasma state the resistance becomes much less. Thus, as the magnesium oxide melts and vaporizes, eventually a current path of relatively low resistance is formed from the discontinuity through the gaseous and molten liquid MgO to the grounded metallic sheath. At this point, the arc transfers from one or both break-created ends of the heating element to the sheath. The force generated by this phenomenon may in some cases actually create a separation in the outer sheath. Once it starts, such a separation in the outer sheath travels lengthwise along the sheath. This phenomenon is commonly referred to as "zippering" because the propagation of the separation along the outer sheath resembles the opening of a zipper.
One device for rapidly terminating "zippering," should it occur in a heating element of the type employed in a dishwasher, is disclosed in commonly assigned U.S. Pat. No. 4,044,224 issued to Jenkins and Herbst. The Jenkins and Herbst device replaces the direct electrical connection between the outer conductive sheath and ground with a fusible link. During normal operation of the heating unit when the insulation material is intact, substantially no current flows through the outer sheath ground connection (with the exception of a small amount of AC leakage current largely the result of capacitive effects). However, when a fault occurs, sufficient current flows through the outer sheath ground connection, to cause the fusible link to open, thereby interrupting the fault current. Additional examples of switching and fuse arrangements responsive to changes in current supplied to the heating unit occasioned by ground faults may be found in commonly-assigned U.S. Pat. No. 4,054,857 to Bowling; and commonly-assigned, co-pending U.S. patent application Ser. No. 282,574 to Wellman and Hornung, as well as British Pat. No. 946,623 to Ryder. Each of these latter references discloses current actuated switching arrangements employed in ranges and cooking appliances to disconnect power from the heating unit in the event of a ground fault.
Arrangements of the foregoing type provide generally satisfactory results when the heating element and associated protective circuitry are properly connected to the power supply; that is, with the power line and neutral line connected to the intended appliance terminals. When so connected, the current actuated switch or fusible link is between the heating element and the hot or power line so that if fault current opens the switch the power line is effectively decoupled from the heating element. However, should such connections be reversed, the switch would only interrupt current flow in the return path to the power supply by decoupling the neutral line, with the result that power is not removed from the heating element and, except in the Jenkins and Herbst arrangement, the ground fault current would proceed undiminished.
Reversed connections may occur from time to time as a result of improper house wiring, improper appliance installation, and possibly errors in the internal wiring of the appliance. The present invention provides a simple, reliable, inexpensive protective circuit arrangement for such heating units which effectively interrupts ground fault conditions and removes power from the heating element regardless of which way the connections to the power supply are made.